1. Field of the Invention
The invention relates to a method and a device for limiting the torque of dental and surgical handpieces.
2. Description of the Related Art
Several surgical procedures, especially in regard to tooth implantation in the jaw area, are known in which the implant members which are provided with a thread are to be screwed into pre-drilled holes in the bone. (In this context, the term xe2x80x9cimplant memberxe2x80x9d is generally meant to include, for reasons of simplification, implants, implant carriers as well as closure screws or supra structure screws.) For this purpose, it is necessary to limit the torque of the electrically operated tool, with which the tightening or screwing action is carried out, to a preset limit value in order to reliably prevent a penetration depth that is too deep, damage of the bone mass, or stripping of the thread in the bone which is usually being cut by the thread of the implant member.
For example, immediately after positioning the implant, closure screws are screwed in and, when working in the jaw area, the gums are closed above the implant and allowed to heal. After approximately half a year or even longer, the closure screw is removed and an implant supra structure is connected to the implant by means of a supra structure screw. This supra structure screw must be screwed or tightened to such an extent that the supra structure screw (for example, an artificial tooth) is never seated loosely within the implant; however, on the other hand, the supra structure screw may never pull the implant out of the bone during tightening.
Based on practical experience and examinations of patients, the maximum torque to be applied is determined, and one is then presented with the problem of not exceeding this torque by the employed tool, for example, an angle piece.
In such a known method, the correlation between the torque and the motor current, provided by the manufacturer, is used in order to either limit the current or switch off the current to the motor and thereby terminate the screwing process when reaching the set point value, which is determined by continuously measuring the motor current. In this method, it has been shown to be disadvantageous that the aging process of the tool, by means of insufficient or excessive lubrication, by wear of the mechanical gear, or possibly by exchanging the mechanical drive, significant deviations of the actually applied torque from the pre-adjusted torque will result for the same motor current.
This has the consequence that the implant members are either tightened too tightly, which may cause all of the aforementioned problems and injuries, or too loosely, which may result in not only the implant carrier but also the implant mounted thereon becoming loose or even detaching, and a treatment of the patient must be repeated, if this is even possible.
Another occurring danger is the so-called too-loose screwing action of such implant parts, be it as a result of incorrectly calibrated handpieces or as a result of the incorrectly estimated torque requirement.
It is an object of the present invention to prevent these risks and disadvantages and to provide a method and a device with which the determined maximum torque can be reliably and safely observed. In a preferred variant it is also desired to monitor when a set point minimal torque is reached.
In accordance with the present invention, this is achieved in that, immediately before the screwing of the implant or implant carrier by means of a surgical handpiece, the motor current required for the application of the set point torque is determined and that, by means of the motor control, the motor current is limited to this value in the subsequent screwing action.
Since the measurement is carried out immediately before the screwing action, all tolerances and imprecisions can be compensated since the same conditions are indeed present during the measurement as well as the immediately following treatment. The term xe2x80x9cimmediately precedingxe2x80x9d or xe2x80x9cimmediately followingxe2x80x9d is understood in the context of the invention such that between the measurement and the treatment no cleaning, lubricating, or servicing tasks and, in particular, no disassembly of the handpiece are carried out.
The device according to the invention comprises a device for applying a constant adjustable braking (tensioning) torque, preferably in the form of a hysteresis tension device, and a device for measuring the motor current so that the surgical handpiece is loaded during the calibration with the set-point torque and the correlation between the set point torque and the motor current is determined unequivocally.
Such hysteresis tension devices are known, for example, from the textile industry, the paper industry, the foil manufacturing industry, the measuring technology, and various other fields of application and are commercially available.
The function of such hysteresis tension devices is based on the magnetic force effect of poles attracting one another in synchronous running and on constant magnetic reversal of a permanent magnet material in the slip area. Manufacturers of such devices are, for example, MOBAC GmbH, Kiel, Germany, or ZF Friedrichshafen AG, Friedrichshafen, Germany.